1. Field of the Invention
The present invention relates to a system for automatically controlling the distance between vehicles on the same track.
2. Description of Related Art
A conventional system for automatically controlling the distance between vehicles is disclosed in CH-A-426 923. A drive device and a control device with three parallel branches is present on each vehicle. The first branch has a DC voltage source with a series-connected working resistor, the second branch has an engine controller for controlling the drive device and the third branch has a monitoring device.
These branches are connected via a first rear sliding contact to a neutral rail running parallel to the carriageway of the vehicles. The branches are also connected via a further sliding contact, which when viewed in the direction of travel is at the front of the vehicle, to a control rail also running parallel to the carriageway.
The control rail is short-circuited to the neutral rail via a third, sliding contact at the rear of each vehicle. The control rail is divided into equally long sections whose length is shorter than the spacing between the front and rear sliding contact of a vehicle.
Between every two successive sections is connected a diode whose conducting direction corresponds to the direction of travel. These diodes form, together with the control rail, a recurrent network.
A positive voltage generated by the voltage source is applied via the corresponding front sliding contact to the control rail. This voltage drops in a step-like manner along the control rail as a result of the short circuit between the control rail and the neutral rail at the rear end of the vehicle in front. The voltage between the front sliding contact and the neutral rail is a distance variable, and thus indicates the distance from a vehicle in front.
The engine controller controls the speed of the vehicle as a function of this distance variable. The greater the distance to the vehicle in front, the higher the voltage which indicates the distance variable. When the distance between vehicles is large, the distance variable is at a maximum value and the engine controller controls the drive device to the highest permissible travel speed. The lower limit of the control range is at a distance variable which corresponds to the spacing between several sections of the control rail. At this lower limit the vehicle is brought to a complete standstill by braking.
The voltage source is periodically and briefly connected with reversed polarity to the control rail. The monitoring device then tests the voltage between the front and rear sliding contact of a vehicle and triggers emergency braking when this voltage deviates from a set value. As a result, a collision with a moving or stationary vehicle is prevented in the event of faults occurring in a diode of the recurrent network, a broken connection between two diodes, a broken contact to one of the sliding contacts, or other faults which result in failure of the automatic control.
The authorized safety control systems known hitherto for automatic or driverless operation are based either on block systems, as in the case of conventional interlocking control technology; or on vital control computers which are installed at the side of the rail route, receive vital position information from vital vehicle computers, and thus vitally control an "electrical sight running system". In this context, vital implies that the systems themselves detect faults and bring about a safe vehicle status when a fault is detected. In railway signalling technology vital includes the connotations of "fail-safe" and "checked-redundant".
Block systems operate with a rough subdivision of the track into so-called block sections. Systems of this kind are designed so that a train travelling through a block section leaves behind one to two items of safety information. The safety information has a low degree of differentiation, for example in the form of signal positions at red or green, for securing the block on the section of rail. Here, it is disadvantageous that a following train may have to constantly change its speed as a function of the block length, and may unnecessarily have to stop. The entire expenditure in terms of material of a block section control of this design is so high that finer sub-divisions of the block sections is too costly.
Another solution which has been devised more recently using reliable vehicle-side or rail route-side computers requires a costly hazards analysis and safety certification of complex software in multi-channel redundant structures.